Build a 4-camera surveillance system and watch from any web browser.
My basement office/workshop is a fair distance from the front of the house, and I frequently want to check the front porch for mail, UPS deliveries, and the arrival of friends. I have experimented with various ways of monitoring the activity there and have found none as useful as a real-time video feed.
Getting video from a camera on the front porch to a screen in the basement is a trivial undertaking, but as projects like this are apt to do, it grew into a $200 four-camera, full-color system that can be monitored in my workshop or from any web browser. Some commercial systems can do the same thing, but they cost much more or use only black-and-white cameras. Meanwhile, I learned that switching composite video signal is actually very easy, doable with a 35¢ CD4066 analog switch chip.